An apparatus comprising a frontend configured to convert an optical IM signal associated with a remote optical transmitter into a plurality of analog electrical signals, determine a plurality of dc offsets for the analog electrical signals, remove the dc offsets from the analog electrical signals to produce a plurality of dc-free analog signals, and convert the dc-free analog signals into a plurality of dc-free digital signals, and a DSP unit coupled to the frontend and configured to perform fiber dispersion compensation on the dc-free digital signals according to a dispersion value associated with the remote optical transmitter to produce a plurality of dc-free compensated digital signals, and add the dc offsets to the compensated digital signals to produce a plurality of dc-restored compensated digital signals.
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16. A method comprising:
converting an optical intensity-modulated (IM) signal associated with a remote optical transmitter into a plurality of analog electrical signals;
determining a plurality of direct current (dc) offsets for the analog electrical signals;
removing the dc offsets from the analog electrical signals to produce a plurality of dc-free analog signals;
converting the dc-free analog signals into a plurality of dc-free digital signals;
performing fiber dispersion compensation on the dc-free digital signals according to a dispersion value associated with the remote optical transmitter to produce a plurality of dc-free compensated digital signals; and
adding the dc offsets to the dc-free compensated digital signals to produce a plurality of dc-restored compensated digital signals.
1. An apparatus comprising:
a frontend configured to:
convert an optical intensity-modulated (IM) signal associated with a remote optical transmitter into a plurality of analog electrical signals;
determine a plurality of direct current (dc) offsets for the analog electrical signals;
remove the dc offsets from the analog electrical signals to produce a plurality of dc-free analog signals; and
convert the dc-free analog signals into a plurality of dc-free digital signals; and
a digital signal processing (DSP) unit coupled to the frontend and configured to:
perform fiber dispersion compensation on the dc-free digital signals according to a dispersion value associated with the remote optical transmitter to produce a plurality of dc-free compensated digital signals; and
add the dc offsets to the dc-free compensated digital signals to produce a plurality of dc-restored compensated digital signals.
11. A method for use in an optical communication device, comprising:
receiving a first optical signal from a first remote optical transmitter;
receiving a second optical signal from a second remote optical transmitter;
converting the first optical signal into a plurality of analog electrical signals comprising two orthogonal polarization components, an X-polarization component and a Y-polarization component, wherein the X-polarization component comprises a first in-phase (i) component and a first quadrature-phase (Q) component, and wherein the Y-polarization component comprises a second i component and a second Q component;
determining a first direct current (dc) offset for the first i component;
determining a second dc offset for the first Q component;
removing the first dc offset from the first i component to produce one of a plurality of first digital electrical signals;
removing the second dc offset from the first Q component to produce another of the first digital electrical signals;
converting the second optical signal into a plurality of second digital electrical signals;
compensating the first digital electrical signals according to a first dispersion value associated with the first remote optical transmitter to produce a plurality of first compensated digital electrical signals;
compensating the second digital electrical signals according to a second dispersion value associated with the second remote optical transmitter to produce a plurality of second compensated digital electrical signals;
adding the first dc offset to one of the first compensated digital electrical signals associated with the first i component; and
adding the second dc offset to another of the first compensated digital electrical signals associated with the first Q component.
2. The apparatus of
3. The apparatus of
4. The apparatus of
perform the fiber dispersion compensation on a first of the dc-free digital signals and a second of the dc-free digital signals associated with the first polarization component; and
perform the fiber dispersion compensation on a third of the dc-free digital signals and a fourth of the dc-free digital signals associated with the second polarization component.
5. The apparatus of
compute a first power signal from a first of the dc-restored compensated digital signals and a second of the dc-restored compensated digital signals associated with the first polarization component;
compute a second power signal from a third of the dc-restored compensated digital signals and a fourth of the dc-restored compensated digital signals associated with the second polarization component; and
add the first power signal and the second power signal to produce a compensated IM signal power.
6. The apparatus of
measure a first dc offset for the first i component; and
measure a second dc offset for the first Q component.
7. The apparatus of
add the first dc offset to a first of the dc-free compensated digital signals associated with the first i component to produce a first of the dc-restored compensated digital signals; and
add the second dc offset to a second of the dc-free compensated digital signals associated with the first Q component to produce a second of the dc-restored compensated digital signals.
8. The apparatus of
compute a static frequency domain filter to provide a dispersion effect corresponding to the dispersion value; and
filter the dc-free digital signals with the static frequency domain filter in a frequency domain.
9. The apparatus of
10. The apparatus of
12. The method of
computing a first power signal for the second X-polarization component;
computing a second power signal for the second Y-polarization component; and
adding the first power signal and the second power signal to produce a compensated intensity-modulated (IM) signal power.
13. The method of
assigning a first transmission time slot to the first remote optical transmitter;
assigning a second transmission time slot next to the first transmission time slot to the second remote optical transmitter; and
inserting a guard interval (GI) between the first transmission time slot and the second transmission time slot, wherein the GI comprises a time duration greater than a mean value of a first pulse broadening duration associated with the first dispersion value and a second pulse broadening duration associated with the second dispersion value.
14. The method of
assigning a first transmission time slot to the first remote optical transmitter;
assigning a second transmission time slot to the second remote optical transmitter;
assigning a third transmission time slot to a third remote optical transmitter; and
arranging the first transmission time slot, the second transmission time slot, and the third transmission time slot based on transmission wavelengths associated with the first remote optical transmitter, the second remote optical transmitter, and the third remote optical transmitter.
15. The method of
17. The method of
18. The method of
performing the fiber dispersion compensation on a first of the dc-free digital signals and a second of the dc-free digital signals associated with the first polarization component; and
performing the fiber dispersion compensation on a third of the dc-free digital signals and a fourth of the dc-free digital signals associated with the second polarization component.
19. The method of
computing a first power signal from a first of the dc-restored compensated digital signals and a second of the dc-restored compensated digital signals associated with the first polarization component;
computing a second power signal from a third of the dc-restored compensated digital signals and a fourth of the dc-restored compensated digital signals associated with the second polarization component; and
adding the first power signal and the second power signal to produce a compensated IM signal power.
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Not applicable.
Optical access networks may be employed to deliver a wide variety of services, such as fiber to the home (FTTH), fiber to the building (FTTB), enterprise connectivity, business connectivity, and mobile back-haul and front-haul for fourth generation (4G) and/or next generation wireless communication. Continuous demands for higher network capacities and greater distance coverages pose challenges in current and future optical access network designs. For example, the effect of fiber transmission impairments may become more severe as the optical transmission speed and the transmission distance increase. Fiber transmission impairments may include chromatic dispersion (CD), polarization mode dispersion (PMD), phase noise, and non-linear effects. However, CD may be one of the most performance limiting factors, especially for high-speed transmissions at long distances.
CD may cause different spectral components (e.g., wavelengths) in an optical signal to travel through an optical fiber at different speeds and arrive at a receiver at different time instants (e.g., with different delays), and thus may temporally broaden the optical pulses that carry the data and lead to inter-symbol interference (ISI). Some systems may compensate CD in a fiber by employing another fiber of opposite-sign dispersion, but may be at the expense of increased loss, complexity, and cost. Recent advances in high-speed analog-to-digital converters (ADCs), high-speed digital-to-analog converters (DACs), and high-performance digital signal processors (DSPs) have enabled fiber-optic impairments to be compensated digitally by DSPs.
The dispersion effect experienced by an optical signal when traveling through a given optical fiber link may be compensated through dispersion post-compensation by an amount that is nominally the opposite of the fiber link dispersion. However, in typical optical access networks, optical network units (ONUs) may send time-division-multiplexed (TDM) signal blocks to an optical line terminal (OLT) at a central office, where the ONUs may be located at different distances away from the OLT. As such, the TDM blocks that are originated from different ONUs may experience different fiber link dispersions, and thus the OLT may not employ the same fiber dispersion post-compensation for all the TDM blocks. In addition, intensity modulation may be commonly employed at the ONUs' transmitters, thus the OLT may require an optical transmitter-specific dispersion post-compensation scheme that is suitable for compensating intensity modulated signals.
In one embodiment, the disclosure includes an apparatus comprising a frontend configured to convert an optical intensity-modulated (IM) signal associated with a remote optical transmitter into a plurality of analog electrical signals, determine a plurality of direct current (DC) offsets for the analog electrical signals, remove the DC offsets from the analog electrical signals to produce a plurality of DC-free analog signals, and convert the DC-free analog signals into a plurality of DC-free digital signals, and a DSP unit coupled to the frontend and configured to perform fiber dispersion compensation on the DC-free digital signals according to a dispersion value associated with the remote optical transmitter to produce a plurality of DC-free compensated digital signals, and add the DC offsets to the compensated digital signals to produce a plurality of DC-restored compensated digital signals.
In another embodiment, the disclosure includes a method for use in an optical communication device comprising receiving a first optical IM signal from a first remote optical transmitter, receiving a second optical IM signal from a second remote optical transmitter, converting the first optical IM signal into a plurality of first digital electrical signals, converting the second optical IM signal into a plurality of second digital electrical signals, compensating the first digital electrical signals according to a first dispersion value associated with the first remote optical transmitter to produce a plurality of first compensated digital electrical signals, and compensating the second digital electrical signals according to a second dispersion value associated with the second remote optical transmitter to produce a plurality of second compensated digital electrical signals.
In yet another embodiment, the disclosure includes a method for use in an optical communication device comprising receiving a transmission schedule comprising a plurality of transmission time slots, wherein the transmission schedule provides an assigned time slot to the device, and wherein the transmission schedule comprises a guard interval (GI) that is based on a CD associated with the device, converting a digital signal sequence into an optical signal by performing an intensity modulation; and transmitting the optical signal in the assigned time slot.
These and other features will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings and claims.
For a more complete understanding of this disclosure, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts.
It should be understood at the outset that, although an illustrative implementation of one or more embodiments are provided below, the disclosed systems and/or methods may be implemented using any number of techniques, whether currently known or in existence. The disclosure should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, including the exemplary designs and implementations illustrated and described herein, but may be modified within the scope of the appended claims along with their full scope of equivalent.
The demand for high-speed, wide-coverage optical access has been growing rapidly. Some research may focus on mitigating the performance limiting CD effect in long-distance optical fiber links. CD compensation may be performed at a direction detection receiver or a coherent detection receiver. Direct detection may refer to the recovery of amplitude information from a received optical carrier signal, whereas coherent detection may refer to the recovery of amplitude and phase information from the received optical carrier signal. For example, a direct detection receiver may perform CD compensation as described in P. Ossieur, et al., “A 10G linear burst-mode receiver supporting electronic dispersion compensation for extended-reach optical links,” Optics Express, Vol. 19, No. 26, Dec. 12, 2011 (Ossieur), which is incorporated herein by reference, in which the CD compensation is performed via a 9-tap feedforward equalizer (FFE) and a 4-tap decision feedback equalizer (DFE). As shown in Ossieur, by performing the CD compensation at the direct detection receiver, the CD-limited transmission distance for a 10 gigabits per second (Gbps) signal may be extended by about 20 percent (%). However, the extended distance may not be sufficient to support a 40 Gbps transmission rate over a distance of about 10 kilometers (km).
Alternatively, a coherent detection receiver may perform CD compensation as described in D. Lavery, et al., “Digital coherent receivers for long-reach optical access networks,” Journal of Lightwave Technology, Vol. 13, No. 4, Feb. 15, 2013, which is incorporated herein by reference, in which optical network units (ONUs) may transmit polarization-division multiplexed quadrature phase-shift keying (PDM-QPSK) signals and an optical line terminal (OLT) may employ a coherent detection receiver to compensate the PDM-QPSK signals for CD. However, current ONUs may employ conventional optical transmitters that are built for intensity modulation, which may refer to the use of light intensity to carry information, for example, by varying the optical power of an optical signal. Thus, the employment of PM-QPSK modulation may increase the complexity and the cost of current ONUs. In addition, PDM-QPSK transmitters may require additional optical amplifiers due to the higher loss associated with PDM-QPSK modulation, and thus may further increase the cost and the size of current ONUs.
Disclosed herein are techniques for performing post-EDC at an optical receiver according to a transmitter-specific or link-dependent fiber dispersion effect. The disclosed post-EDC techniques may be applied to optical IM signals that are transmitted by conventional optical transmitters for removing fiber dispersion effect. The disclosed post-EDC techniques may operate on the electric (E)-field of a received optical IM signal and may be suitable for signals with any modulation format, such as on-off keying (OOK), n-level PAM (n-PAM), orthogonal frequency-division multiplexing (OFDM), discrete multi-tone (DMT), duobinary, differential phase-shift keying (DPSK), differential quadrature phase-shift keying (DQPSK), or other modulation formats that may be generated by simple transmitters. In addition, the disclosed post-EDC techniques may be applicable to polarization-division multiplexed (PDM) signals. In an embodiment, an OLT may determine an amount of fiber dispersion for each connected ONU and may perform post-EDC on each received US signal to remove about all the fiber dispersion effect in an optical transport link between the OLT and the ONU that transmitted the US signal, where US may refer to the direction in which data may be transferred from the ONUs to the OLT 110. To further reduce the fiber dispersion effect, the OLT may design US transmission schedules with GIs inserted between adjacent US transmission time slots. The disclosed embodiments may implement the post-EDC at a coherent optical receiver comprising a polarization-diversity optical hybrid coupled to a DSP unit. The polarization-diversity optical hybrid may be configured to convert a received optical IM signal into two orthogonal polarizations. The DSP unit may be configured to separately perform the post-EDC on each polarization and reconstruct the IM signal after the post-EDC. The coherent optical receiver may further comprise a DC controller configured to measure and remove DC offsets in the analog electrical domain, and the DSP unit may be further configured to restore the DC offsets when reconstructing the IM signal. By removing the DC offsets in the analog electrical domain, the disclosed embodiments may employ lower resolutions ADCs and DSPs. The disclosed embodiments may provide wide-coverage (e.g., up to about 100 km), high-speed (e.g., greater than about 10 Gbps) optical access with close to zero dispersion and may enable the ONUs to reuse existing resources or to upgrade with relatively small modifications. It should be noted that the present disclosure may describe the embodiments in the context of an OLT, but the disclosed embodiments may be applicable to receivers in ONUs or any other optical communication devices.
The OLT 110 may be any device configured to communicate with the ONUs 120 and another backbone network (e.g., the Internet). Specifically, the OLT 110 may act as an intermediary between the backbone network and the ONUs 120. For instance, the OLT 110 may forward data received from the backbone network to the ONUs 120, and forward data received from the ONUs 120 onto the backbone network. Although the specific configuration of the OLT 110 may vary depending on the type of PON 100, the OLT 110 may comprise an optical transmitter and an optical receiver. When the backbone network employs a network protocol, such as Ethernet or synchronous optical networking/synchronous digital hierarchy (SONET/SDH), that is different from the PON protocol used in the PON 100, the OLT 110 may comprise a converter that may convert the network protocol into the PON protocol. The OLT 110 converter may also convert the PON protocol into the network protocol. The OLT 110 may be located at a central location, such as a central office, but may be located at other locations as well.
The ODN 130 may be a data distribution system. For example, the ODN 130 may comprise a feeder fiber 131, a plurality of drop fibers 132, 133, and 134, and a splitter 135 that couples the feeder fiber 131 to the drop fibers 132, 133, and 134. The feeder fiber 131 and the drop fibers 132, 133, and 134 may be any optical fiber cables that transport optical signals. The splitter 135 may be any optical coupler (e.g., a directional coupler or a multi-mode interference (MMI) coupler) configured to split an optical signal into a plurality of portions, each carried via one of the drop fibers 132, 133, and 134. The ODN 130 may further comprise other distributors, couplers, and/or other equipment. As shown in
The ONUs 120 may be any devices configured to communicate with the OLT 110 and a customer or an end-user. Specifically, the ONUs 120 may act as an intermediary between the OLT 110 and the customer. For instance, the ONUs 120 may forward data received from the OLT 110 to the customer, and forward data received from the customer to the OLT 110. Although the specific configuration of the ONUs 120 may vary depending on the type of PON 100, the ONUs 120 may comprise an optical transmitter configured to send optical signals to the OLT 110 and an optical receiver configured to receive optical signals from the OLT 110. Additionally, the ONUs 120 may comprise a converter that converts the optical signal into electrical signals for the customer, such as signals in the Ethernet or asynchronous transfer mode (ATM) protocol, and a second transmitter and/or receiver that may send and/or receive the electrical signals to and from a customer device. The ONUs 120 may be located at distributed locations, such as the customer premises, but may be located at other locations as well.
In some embodiments, the PON 100 may be coupled to a coaxial network to form a hybrid access network, such as an Ethernet PON over coaxial (EPoC) network. In a hybrid access network, a coaxial line terminal (CLT) may act as an intermediary between the OLT 110 and coaxial network units (CNUs) that are connected to the coaxial network. The CLT may forward data received from the OLT 110 to the CNUs and forward data received from the CNUs to the OLT 110.
Each of the optical transport links 161, 162, and 163 may comprise a fiber dispersion effect due to the use of fibers 131, 132, 133, and 134 as transmission media and the different propagation velocities among different signal components along the fibers 131, 132, 133, and 134. For example, an ONU 120 may transmit an optical signal comprising a plurality of optical pulses, each carrying a data symbol, to the OLT 110. When the optical signal propagates along the optical transport links 161, 162, or 163, the CD effect may cause the optical pulses to spread in time, which may be referred to as the pulse broadening effect. Thus, the OLT 110 may receive adjacent optical pulses that are overlapped and adjacent data symbols that interfere with each other, which may be referred to as the ISI or the CD-induced interference. As such, the OLT 120 may not be able to recover the original data without error or may not recover the original data at all when the ISI is substantially strong. In order to reverse or remove the CD effect, the OLT 110 may compensate the received signal to account for the CD effect prior to performing data recovery. It should be noted that the CD effect may increase with fiber lengths and data speeds (e.g., baud rates).
Each of the optical transport links 161, 162, and 163 may comprise a different CD effect due to different fiber lengths, and thus the OLT 110 may receive US signals with different CD effects depending on the originating ONUs. One approach to compensating the US signals may be to account for the CD effect according to the originating ONUs 120. For example, each optical transport link 161, 162, and 163 may comprise an amount of CD represented by D1, D2, and DN, respectively. When the OLT 110 receives a US signal from the ONU1 120 connecting to the OLT 110 via the optical link 161, the OLT 110 may compensate the US signal for a D1 amount of CD. Similarly, when the OLT 110 receives a US signal from the ONU2 120 connecting to the OLT 110 via the optical link 162, the OLT 110 may compensate the US signal for a D2 amount of CD, and when the OLT 110 receives a US signal from the ONUN 120 connecting to the OLT 110 via the optical link 163, the OLT 110 may compensate the US signal for a D3 amount of CD. After the compensation, the US signals may comprise close to zero residual CD. The compensation may be referred to as post-EDC since the OLT 110's receiver may compensate a signal for CD in the electrical domain after the signal propagates through a dispersion-limited fiber link, such as the optical transport links 161, 162, or 163.
The OLT 110 may obtain the amount of CD in the optical transport links 161, 162, and 163 via several mechanisms. For example, the CD effect may be measured by test equipment during an initial set up or installation of an ONU 120 and input to the OLT 110, where the test equipment may send a test signal to the ONU 120 and measure the delay in the returned signal. Alternatively, the OLT 110 may measure the amount of CD when the ONU 120 joins the network during a network discovery phase by iteratively adjusting the amount of CD for pre-compensation and determining an optimum estimate for the amount of CD.
The OLT 110 may perform dynamic bandwidth allocation to assign US transmission bandwidths and DS transmission bandwidths to the ONUs 120. In an embodiment, the OLT 110 may employ a time-division multiple access (TDMA) scheme for US transmission. In the TDMA scheme, the US channel or transmission bandwidth may be shared among the ONUs 120 by dividing the US channel into a plurality of time slots 141, 142, 143, 144, 145, and 146, each designated for one of the ONUs 120. For example, the OLT 110 may assign the time slots 141 and 145 to a first of the ONUs 120 (e.g., ONU1), the time slots 142 and 144 to a second of the ONUs 120 (e.g., ONU2), and the time slots 143 and 146 to a third of the ONUs 120 (e.g., ONUN). Each ONU 120 may transmit US data according to the time slot assignments. It should be noted that the OLT 110 may employ other access schemes, such as a wavelength-division multiple access (WDMA) scheme separately or in combination with the TDMA scheme to communicate with the ONUs 120. When the OLT 110 employs a WDM scheme in combination with the TDMA scheme, the OLT 110 may sort the ONUs 120 by wavelengths for CD compensation.
In addition to compensating DS signals to account for fiber dispersion, the OLT 110 may insert GIs 150 between each of the time slots 141-146 to further reduce the CD-induced interference, where the GIs 150 may be larger than the CD-induced pulse broadening duration, ΔTCD. For example, a symbol period may be represented by TS, which may be based on the data speed or the baud rate, and the CD-induced interference may cause the symbol to span a period, TCD, that is greater than TS, where the CD-induced pulse broadening duration, ΔTCD, may be referred to as the difference between TCD and TS (e.g., ΔTCD=TCD−TS).
The OLT 110 may insert the GIs 150 with different durations between adjacent timeslots 141-146 according to the ONUs 120 that are assigned to the adjacent time slots. For example, each GI 150 may be configured to be a value larger than about 50% of the sum of the pulse broadening durations of the ONUs 120 that are assigned to the adjacent time slots as follows:
GI>0.5×(ΔTCCD(i)+ΔTCCD(i+1)), (1)
where ΔTCCD (i) may represent the pulse broadening duration for a first ONU 120 assigned to a time slot i and ΔTCCD(i+1) may represent the pulse broadening duration for a second ONU 120 assigned to a time slot i+1 subsequent to the time slot i.
Alternatively, all GIs 150 may be configured with a same duration that is larger than a mean of the pulse broadening durations for all ONUs 120 as follows:
GI>mean(ΣiΔTCCD(i)). (2)
It should be noted that the OLT 110 may assign US transmission bandwidths and DS transmission bandwidths to the CLT by employing substantially similar scheduling mechanisms as for the ONUs 120. In addition, the OLT 110 may employ similar post-EDC mechanisms to compensate US signals received from the CLT.
The LO laser 210 may be configured to provide a reference signal 201 with a phase and/or frequency substantially close to the phase and/or the frequency of the transmitter. The polarization-diversity optical hybrid 220 may be coupled to the LO laser 210 and configured to mix the optical signal 205 with the reference signal 201 and convert the optical signal 205 into a baseband signal. For example, the polarization-diversity optical hybrid 220 may comprise a first polarization beam splitter (PBS) 221, a second PBS 222, a first 90 degree (°) hybrid 223, and a second 90° hybrid 224.
The PBSs 221 and 222 may be optical devices configured to split an optical signal into two orthogonal polarization components, for example, an X-polarization component and a Y-polarization component. The PBS 221 may be coupled to the first 90° hybrid 223 and the PBS 222 may be coupled to the second 90° hybrid 224. The first 90° hybrid 223 and the second 90° hybrid 224 may be optical devices, such as 2×2 Mach-Zehnder Interferometers (MZIs), each configured to mix an incoming signal, such as the optical signal 205, with a reference signal, such as the reference signal 201, to produce four signals. The signals may be in the complex-field space, comprising a real signal sum component, a real signal difference component, an imaginary signal sum component, and an imaginary signal difference component.
As shown in
Each balance detector 230 may comprise a pair of photodetectors 231 coupled to a signal difference element 232. Each photodetector 231 may be a photodiode configured to convert an optical signal into an analog electrical signal. Each signal difference element 232 may provide a signal difference between the analog electrical signals converted by the respective pair of photodetectors 231, for example, cancelling noise fluctuations from the received optical signal 205 and the reference signal 201. As shown in
The DC controller 240 may comprise a DC offset monitor 241 and a plurality of DC blocking elements 242. The DC offset monitor 241 may be configured to monitor and measure a DC offset for each of the electrical signal components, IX, QX, IY, and QY. DC offset may be referred to as the mean value of a signal waveform or the mean voltage offset with respect to zero volts (V). For example, each of the electrical signal components, IX, QX, IY, and QY, may comprise a DC offset, DCI
The DSP unit 260 may comprise a plurality of signal adders 261, a plurality of post-EDC units 262, a plurality of DC adders 263, a matched filter 264, an IM signal reconstruction unit 265, and a demodulator 266. Each signal adder 261 may be configured to add an I component and a Q component in the complex-field space. For example, a first of the signal adders 261 may be configured to add the I component and the Q component in the X-polarization to produce a DC-free X-polarization complex signal, denoted as EX′, and a second of the signal adders 262 may be configured to add the I component and the Q component in the Y-polarization to produce a DC-free Y-polarization complex signal, denoted as EY′.
The post-EDC units 262 may be coupled to the signal adders 261 and configured to perform post-EDC. For example, a first of the post-EDC units 262 may process EX′ to produce a DC-free compensated X-polarization signal, EX″, and a second of the post-EDC units 262 may process EY′ to produce a DC-free compensated Y-polarization signal, denoted as EY″. The two post-EDC units 262 may be configured to compensate CD based on, for example, a CD amount of D ps/nm, associated with the transmitter of the received optical signal 205 and may operate in a frequency domain. For example, each post-EDC unit 262 may comprise a fast Fourier transform (FFT) sub-unit, a frequency filter, and an inverse FFT (IFFT) sub-unit. The FFT sub-unit may convert EX′ or EY′ into a frequency domain sequence. The frequency filter may be configured to comprise a phase matched to an opposite value of D, for example, by setting a post-compensation value, Dpost, to −D. The frequency domain sequence may be filtered by the frequency filter to reverse or remove the CD effect. After filtering, the IFFT sub-unit may convert the compensated frequency sequence into a time domain sequence. The post-EDC units 262 may be coupled to the DC adders 263.
The DC adders 263 may be configured to restore the DC offsets removed in the DC controller 240 by adding the respective DC offsets to the DC-free compensated X-polarization and Y-polarization signals. For example, a first of the DC adders 263 may be configured to add the DC offsets DCI
real(EX′″)=real(EX″)+DCI
imag(EX′″)=imag(EX″)+DCQ
where real represents the real component and imag represents the imaginary component.
Similarly, a second of the DC adders 263 may be configured to add the DC offsets DCI
real(EY′″)=real(EY″)+DCI
imag(EY′″)=imag(EY″)+DCQ
The matched filter 264 may be coupled to the DC adders 263. The matched filter 264 may be configured to perform pulse-shaping to match EX′″ and EY′″ to the transmit pulse-shaping employed at the transmitter of the optical received signal 205. The outputs of the matched filter 264 may be represented as EX and EY. It should be noted that the matched filter 264 may be optional depending on the modulation scheme in use, for example, an OFDM modulated signal may be processed without a matched filter, where EX′″ and EX′″ may be the same signal, and EY′″ and EY′″ may be the same signal. The matched filter 264 may be coupled to the IM signal reconstruction unit 265.
The IM signal reconstruction unit 265 may be configured to reconstruct the E-field of the optical signal 205 after the post-EDC. For example, the reconstructed power waveform of the IM signal may be computed as shown below:
P(t)=|EX(t)|2+|EY(t)|2, (5)
where |EX(t)|2 represents the power signal for EX, |EY(t)|2 represents the power signal for EY, and P(t) represents the power waveform of the IM signal and t represents time.
The demodulator 266 may be coupled to the IM signal reconstruction unit 265 and may be configured to demodulate the CD compensated IM signal according to a pre-determined modulated format, such as OOK, n-PAM, duobinary, DMT, or OFDM, to recover the original transmitted data.
It should be noted that the optical receiver 200 may be referred to as an alternating current (AC)-coupled digital coherent detection (DCD) receiver since the DC offsets are removed prior to the digital coherent detection at the DSP unit 260.
It is understood that by programming and/or loading executable instructions onto the transceiver unit 300, at least one of the processing unit 330 and/or memory module 332 are changed, transforming the transceiver unit 300 in part into a particular machine or apparatus, e.g., a multi-core forwarding architecture, having the novel functionality taught by the present disclosure. It is fundamental to the electrical engineering and software engineering arts that functionality that can be implemented by loading executable software into a computer can be converted to a hardware implementation by well-known design rules. Decisions between implementing a concept in software versus hardware typically hinge on considerations of stability of the design, numbers of units to be produced, and/or clock speed requirements rather than any issues involved in translating from the software domain to the hardware domain. Generally, a design that is still subject to frequent change may be preferred to be implemented in software, because re-spinning a hardware implementation is more expensive than re-spinning a software design. Generally, a design that is stable that will be produced in large volume may be preferred to be implemented in hardware, for example in an ASIC, because for large production runs the hardware implementation may be less expensive than the software implementation. Often a design may be developed and tested in a software form and later transformed, by well-known design rules, to an equivalent hardware implementation in an ASIC that hardwires the instructions of the software. In the same manner as a machine controlled by a new ASIC is a particular machine or apparatus, likewise a computer that has been programmed and/or loaded with executable instructions may be viewed as a particular machine or apparatus.
It should be understood that any processing of the present disclosure may be implemented by causing a processor (e.g., a general purpose central processing unit (CPU) inside a computer system) in a computer system to execute a computer program. In this case, a computer program product can be provided to a computer or a mobile device using any type of non-transitory computer readable media. The computer program product may be stored in a non-transitory computer readable medium in the computer or the network device. Non-transitory computer readable media include any type of tangible storage media. Examples of non-transitory computer readable media include magnetic storage media (such as floppy disks, magnetic tapes, hard disk drives, etc.), optical magnetic storage media (e.g., magneto-optical disks), compact disc read only memory (CD-ROM), compact disc recordable (CD-R), compact disc rewritable (CD-R/W), digital versatile disc (DVD), Blu-ray (registered trademark) disc (BD), and semiconductor memories (such as mask ROM, programmable ROM (PROM), erasable PROM), flash ROM, and RAM). The computer program product may also be provided to a computer or a network device using any type of transitory computer readable media. Examples of transitory computer readable media include electric signals, optical signals, and electromagnetic waves. Transitory computer readable media can provide the program to a computer via a wired communication line (e.g., electric wires, and optical fibers) or a wireless communication line.
At step 410, a plurality of digital electrical signals may be received, for example, from a plurality of ADCs, such as the ADCs 250. The digital electrical signals may be converted from a received IM signal, such as the optical signal 205, via a frontend, such as the frontend 215. The digital electrical signals may comprise an I component and a Q component of an X-polarization component, and an I component and a Q component of a Y-polarization component corresponding to an ONU, ONU(i). The digital electrical signals may be DC-free signals, where the DC offset may be measured and removed by a DC controller, such as the DC controller 240.
At step 420, oversampling may be performed on the digital electrical signals to increase resolution, for example, via a 2× oversampling. At step 430, after oversampling, post-EDC may be performed on the oversampled digital electrical signals. The X-polarization component and the Y-polarization component may be separately compensated according to a corresponding D(i) associated with the ONU(i) to produce a compensated X-polarization component, EX″, and a compensated Y-polarization component, EY″. Since CD may be dependent on fiber lengths and may be substantially static (e.g., quasi-static), a frequency domain approach may be suitable for the post-EDC.
For example, a plurality of frequency domain filters may be determined such that each frequency domain filter may produce a dispersion effect based on a given compensation dispersion value, Dpost(i), that is opposite to a corresponding ONU-specific dispersion value, D(i) (e.g., Dpost(i)=−D(i)). Each polarization component may be transformed into a frequency sequence via an FFT, a frequency domain filter may be selected according to Dpost(i), and the frequency sequence may be compensated by filtering the frequency sequence with the selected frequency domain filter. The compensated frequency sequence may be converted to a time domain digital signal sequence via an IFFT. In some embodiments, other frequency-response equalization and other non-linearity compensation may be performed. The frequency domain compensation or equalization may be performed by employing an overlap-and-add approach or an overlap-and-save approach. It should be noted that the frequency domain filters may be static filters, but may be updated during a network reconfiguration (e.g., changes in physical configuration, such as fiber length change).
At step 440, the DC offsets may be added to the compensated X-polarization component, EX″, and the compensated Y-polarization component, EY″. For example, the DC offsets may be read or received from the DC controller and the DC offsets may be added accordingly as described above in equations (3) and (4).
At step 450, matched filtering may be performed to produce one sample per symbol, for example, matched to the transmitter's sample timing and pulse-shaping. At step 460, a CD compensated IM signal may be reconstructed, where the power waveform of the compensated IM signal may be computed according to equation (5) as described above. At step 470, demodulation may be performed on the CD compensated IM signal to recover the original transmitted data bits. It should be noted that steps 420 and 450 may be optional.
At step 530, the first optical signal may be converted into a plurality of first digital electrical signals, for example, via a frontend, such as the frontend 215, and a plurality of ADCs, such as the ADCs 250. At step 540, the second optical signal may be converted into a plurality of second digital electrical signals.
At step 550, the first digital electrical signals may be compensated according to a first dispersion value associated with the first remote optical transmitter, for example, by employing similar mechanisms as described in the step 430. At step 560, the second digital electrical signals may be compensated according to a second dispersion value associated with the second remote optical transmitter. For example, the first dispersion value may be opposite to an amount of CD in an optical fiber link, such as the optical transport link 161, 162, or 163, between the first remote optical transmitter and the optical receiver. Similarly, the second dispersion value may be opposite to an amount of CD in an optical fiber link between the second remote optical transmitter and the optical receiver.
At least one embodiment is disclosed and variations, combinations, and/or modifications of the embodiment(s) and/or features of the embodiment(s) made by a person having ordinary skill in the art are within the scope of the disclosure. Alternative embodiments that result from combining, integrating, and/or omitting features of the embodiment(s) are also within the scope of the disclosure. Where numerical ranges or limitations are expressly stated, such express ranges or limitations should be understood to include iterative ranges or limitations of like magnitude falling within the expressly stated ranges or limitations (e.g., from about 1 to about 10 includes, 2, 3, 4, etc.; greater than 0.10 includes 0.11, 0.12, 0.13, etc.). For example, whenever a numerical range with a lower limit, Rl, and an upper limit, Ru, is disclosed, any number falling within the range is specifically disclosed. In particular, the following numbers within the range are specifically disclosed: R=Rl+k*(Ru−Rl), wherein k is a variable ranging from 1 percent to 100 percent with a 1 percent increment, i.e., k is 1 percent, 2 percent, 3 percent, 4 percent, 7 percent, . . . , 70 percent, 71 percent, 72 percent, . . . , 97 percent, 96 percent, 97 percent, 98 percent, 99 percent, or 100 percent. Moreover, any numerical range defined by two R numbers as defined in the above is also specifically disclosed. Unless otherwise stated, the term “about” means±10% of the subsequent number. Use of the term “optionally” with respect to any element of a claim means that the element is required, or alternatively, the element is not required, both alternatives being within the scope of the claim. Use of broader terms such as comprises, includes, and having should be understood to provide support for narrower terms such as consisting of, consisting essentially of, and comprised substantially of. Accordingly, the scope of protection is not limited by the description set out above but is defined by the claims that follow, that scope including all equivalents of the subject matter of the claims. Each and every claim is incorporated as further disclosure into the specification and the claims are embodiment(s) of the present disclosure. The discussion of a reference in the disclosure is not an admission that it is prior art, especially any reference that has a publication date after the priority date of this application. The disclosure of all patents, patent applications, and publications cited in the disclosure are hereby incorporated by reference, to the extent that they provide exemplary, procedural, or other details supplementary to the disclosure.
While several embodiments have been provided in the present disclosure, it should be understood that the disclosed systems and methods might be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented.
In addition, techniques, systems, subsystems, and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, modules, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as coupled or directly coupled or communicating with each other may be indirectly coupled or communicating through some interface, device, or intermediate component whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and could be made without departing from the spirit and scope disclosed herein.
Liu, Xiang, Effenberger, Frank
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| Oct 03 2014 | LIU, XIANG | FUTUREWEI TECHNOLOGIES, INC | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 033929 | /0379 | |
| Oct 03 2014 | EFFENBERGER, FRANK | FUTUREWEI TECHNOLOGIES, INC | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 033929 | /0379 |
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